Embodiments of the present disclosure relate to wind turbines, and more particularly to reducing tower oscillations in wind turbines.
Modern wind turbines operate in a wide range of wind conditions. These wind conditions can be broadly divided into two categories—below rated speeds and above rated speeds. To produce power in these wind conditions, wind turbines may include sophisticated control systems such as pitch controllers and torque controllers. These controllers account for changes in the wind conditions and accompanying changes in wind turbine dynamics. For example, pitch controllers generally vary the pitch angle of rotor blades to account for the changes in wind conditions and turbine dynamics. During below rated wind speeds, wind power may be lower than the rated power output of the wind turbine. In this situation, the pitch controller may attempt to maximize the power output by pitching the rotor blades substantially perpendicular to the wind direction. Alternatively, during above rated wind speeds, wind power may be greater than the rated power output of the wind turbine. Therefore, in this case, the pitch controller may restrain wind energy conversion by pitching the rotor blades such that only a part of the wind energy impinges on the rotor blades. By controlling the pitch angle, the pitch controller thus controls the velocity of the rotor blades and in turn the energy generated by the wind turbine.
Along with maintaining rotor velocity, pitch controllers may also be employed to reduce tower oscillations. Tower oscillations or vibrations occur due to various disturbances, such as turbulence, inefficient damping, or transition between the two wind conditions. Moreover, the tower may vibrate along any degree of freedom. For example, the tower may vibrate in a fore-aft direction (commonly referred to as tower nodding), in a side-to-side direction (commonly referred to as tower naying), or along its longitudinal axis (commonly referred to as torsional vibration).
Tower nodding is usually caused by aerodynamic thrust and rotation of the rotor blades. Every time a rotor blade passes in front of the tower, the thrust of the wind impinging on the tower decreases. Such continuous variation in wind force may induce oscillations in the tower. Moreover, if the rotor velocity is such that a rotor blade passes over the tower each time the tower is in one of its extreme positions (forward or backward), the tower oscillations may be amplified. Typically, the oscillations in the fore-aft direction are automatically minimized due to aerodynamic damping. Aerodynamic damping relies on the fact that the top of the tower constantly oscillates in the fore-aft direction. When the top of the tower moves upwind (or forward), the rotor thrust is increased. This increase in rotor thrust pushes the tower back downwind. The downwind push in turn aids in dampening the tower oscillations. Similarly, when the top of the tower moves downwind, the rotor thrust may be decreased. This decrease in rotor thrust pushes the tower back upwind. The upwind push also aids in dampening the tower oscillations.
Although aerodynamic damping aids in reducing oscillations considerably, if the rotor velocity is synchronized with the tower oscillations, the results may be detrimental for wind turbine components. In such instances, the tower may oscillate at a high rate causing mechanical strain and possible damage to the tower. Moreover, such synchronization may amplify the rotor velocity at tower resonance frequency, thereby potentially damaging generators and/or drivetrains connected to the rotor blades. As the amplification of tower oscillations is dependent on the rotor velocity, pitching the rotor to adjust its velocity may prevent amplification of the tower oscillations. Accordingly, by pitching the rotor blades, the pitch controller may control the rotor velocity and prevent amplification of the tower oscillations.
Typically, the pitch controller utilizes two separate control loops for the two functions—controlling the rotor velocity and reducing the tower oscillations. A rotor velocity control loop is employed to determine a pitch angle to control rotor velocity and a tower-damping control loop is used to compute a pitch angle to reduce tower oscillations. Often, these feedback loops operate relatively independently of each other. For example, the rotor velocity control loop may determine the pitch angle based on rotor velocity, wind speed, and current pitch angle. The tower-damping control loop, on the other hand, may determine the pitch angle based on tower deflection, tower top velocity, tower top acceleration, current pitch angle, and wind speed. Because of this independence, the currently available rotor velocity control loops may compute a pitch angle to maintain rotor speed that may disadvantageously induce tower oscillations instead of reducing them. Moreover, these rotor velocity control loops may cause energy amplification in the rotor near tower resonance frequencies. Such amplification may increase oscillations in the tower and increase the fatigue load placed on the wind turbine. Over time, such fatigue loads may reduce the life of wind turbine parts and increase the expenses associated with wind turbines.